I got a little scratch inside my cheek from my teeth. What should I do?

Check scratch. Sorry to hear that. Your body will heal it up quickly. Nothing special to do. If it swells or worsens over the next few days, see a dentist. If the tooth has a sharp edge, the dentist can smooth it off (or repair the tooth).
Biting cheek? Did the scratch develop over time or did it just occur, and has it been there for awhile. You may be subconsciously biting your cheek. A dentist may conservatively smooth the tooth edges, rounding the tooth/teeth to help. If this is a sore that has remained for a long time, definitely see a dentist and have it checked. Better to be safe.
Little scratch. Usually there are no worries at all, although since our mouths are a bacterial haven they may become infected or may develop a traumatic ulceration. At any rate there are easy treatments for either situation however, they are not likely to happen in most cases.